conftest loads harness/.env once (session, autouse) via python-dotenv, and exposes
an l2_env fixture that SKIPS (not fails) when any L2 prerequisite var is missing/
empty: THOTH_DWH_API_KEY, THOTH_VEC_API_KEY, THOTH_VEC_WRITE_API_KEY, THOTH_SSL_CA.
So the default run (pytest = L0+L1, addopts '-m not l2') stays green without .env;
only pytest -m l2 (pre-release, with .env + VPN) exercises them. l0/l2 markers were
registered in A9. tests/l2/ package created for the L2 tests (D4, D5).
